Solution to Moments Question: Find 11 N Force

[hasnain721]

Homework Statement

http://img510.imageshack.us/img510/7663/26160035im5.jpg

Homework Equations

principle of moments

The Attempt at a Solution

I took moments about the base of the retort stand :
(220 X x) = 228 X 9

x = 9.33 which is wrong as the question says show that it is 11 Newtons.

 

[Doc Al]

Where's the center of mass of the stand?

 

[hasnain721]

Where's the center of mass of the stand?

in the middle i believe where the pivot it

 

[Doc Al]

I mean the center of mass of the retort stand alone. (Its center of mass cannot be over the pivot, otherwise it would tip over when the weight is added.)

 

[hasnain721]

I mean the center of mass of the retort stand alone. (Its center of mass cannot be over the pivot, otherwise it would tip over when the weight is added.)

i thougt that the centre of mass is where the retort stand balances ..this is the full question :


http://img218.imageshack.us/img218/8936/15977211jl9.jpg

 

[Doc Al]

Yes, in this diagram--given in part (b)--the center of mass is over the pivot point.

Now use that information when you find the moments in part (i).

 

[hasnain721]

Yes, in this diagram--given in part (b)--the center of mass is over the pivot point.

Now use that information when you find the moments in part (i).

rite...moments is when u multiply the force by the perpendicular distance:

moment about the pivot :


(220 X x) = (448-220) X 9



this does not work!

 

[Doc Al]

There are two forces that create torques on this thing. One is its weight. Where does that act? (Hint: Not at 220 mm from the pivot! Hint 2: Where's the center of mass of the retort stand?)

 

[hasnain721]

There are two forces that create torques on this thing. One is its weight. Where does that act? (Hint: Not at 220 mm from the pivot! Hint 2: Where's the center of mass of the retort stand?)

the weight acts through the pivot and if we r taking moment about the pivot, i believe that it does not have a moment rite?

 

[Doc Al]

The weight of any object acts through its center of mass. Use the first diagram to find the center of mass of this object. (How far is the center of mass from the left end?)

 

[hasnain721]

The weight of any object acts through its center of mass. Use the first diagram to find the center of mass of this object. (How far is the center of mass from the left end?)

220 mm ?

 

[Doc Al]

220 mm ?

I'm talking about the first diagram in your latest scan--the one marked part (b). 220 mm doesn't appear anywhere on that diagram.

 

[hasnain721]

I'm talking about the first diagram in your latest scan--the one marked part (b). 220 mm doesn't appear anywhere on that diagram.

ah...40mm

 

[Doc Al]

ah...40mm

Right. That diagram tells you where the center of mass of the object is. Now use that fact when you analyze the forces in part (i). Hint: How far is the center of mass from the pivot in that second diagram?

 

[hasnain721]

Right. That diagram tells you where the center of mass of the object is. Now use that fact when you analyze the forces in part (i). Hint: How far is the center of mass from the pivot in that second diagram?

srry I'm tryin my best...i'm rubbish at moments any ways :


if we take moments about the centre of mass : (40 X x) + ((448 - 40) X 9 ) = 0

stil doesn't work.

 

[Doc Al]

Take moments about the pivot point, not the center of mass. (Since the weight acts at the center of mass, it cannot exert a moment about the center of mass!)

 

[hasnain721]

Take moments about the pivot point, not the center of mass. (Since the weight acts at the center of mass, it cannot exert a moment about the center of mass!)

ok...if we take moments about the pivot then :

(40 X x) + (228 X 9 ) = 0 ?

 

[Doc Al]

ok...if we take moments about the pivot then :

(40 X x) + (228 X 9 ) = 0 ?

Two problems:
(1) The distance between center of mass and pivot point is not 40. (That's the distance between center of mass and the left end.)
(2) Since one moment is clockwise and the other counterclockwise, they must have different signs.

 

[hasnain721]

Two problems:
(1) The distance between center of mass and pivot point is not 40. (That's the distance between center of mass and the left end.)

the centre of mass is at 40 mm and the pivot is 220 mm. Therefore the distance between them should be 220 - 40 = 180 mmm

(2) Since one moment is clockwise and the other counterclockwise, they must have different signs.

yea clockwise moments = anticlock wise moments

(180 X x) = ((448 - 220) X 9 )

180 x = 228 X 9

180 x = 2052
x = 11.4 !IT WORKS!THANK U SOOO MUCH!

 

[hasnain721]

man u explain every thing so well... ifu really dun mind can u explain the next one please :
http://img528.imageshack.us/img528/7795/43744261tf4.jpg i don't see how force and weight are related?

 

[Doc Al]

The object is now balanced on its left end, so take moments about that point. Where does its weight act? (Same place as always: Its center of mass.) The "Newtonmeter" just provides an upward force to balance things. Set up your moment equation and you can solve for that force. (Since you know the weight of the object now.)

 

[hasnain721]

The object is now balanced on its left end, so take moments about that point. Where does its weight act? (Same place as always: Its center of mass.) The "Newtonmeter" just provides an upward force to balance things. Set up your moment equation and you can solve for that force. (Since you know the weight of the object now.)

so if we take moments about the left end then :


(40 X 11.4) = ( 448 X x)

x = 0.991

x = 1 N is that rite?

 

[Doc Al]

Looks good to me. (My calculator gives an answer slightly different from 0.991--but close enough!)

 

[hasnain721]

Looks good to me. (My calculator gives an answer slightly different from 0.991--but close enough!)

thank u veeeeeeeeery much...would u recommend any websites which have questions on moments as i am fairly weak in that area. I really appreciate your efforts in volunteering to help dumb people like me.

 

[Doc Al]

Look at some of the examples linked on this page: http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html#equi"

 

[hasnain721]

Look at some of the examples linked on this page: http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html#equi"

thats a lot again!